Selectivity of gill net in mapará cought, Hypophthalmus marginatus, in the reservoir of Tucurui hydropower (Pará, Brazil)

Abstract

The aim of this work was to establish the most selective size of the gill net used to catch mapará, Hypophthalmus marginatus (Valenciennes, 1840). The study was developed in the reservoir of UHE Tucuruí­­ by using experimental fisheries every three months from January 2001 to June 2006. It was used gill net with 50 meters long and 3 meters high with mesh sizes from 40 to 200 mm. All fishes caught were identified, measured and weighed. The selectivity curves were estimated by determination of the selection factor, sample variation and optimum animal size. The mapará fish were caught in gill net with mesh size from 20 to 140 mm (1,424 fishes). The mesh size of 80 mm was responsible for the higher proportion of catch (45.4%). Once confirmed the linear relation between Ln(C_b/C_a) and the class center (L) for gillnet pairs studied, the selectivity curves were estimated. According to these curves, the 80 mm mesh gillnet presents an optimum length of catch equal to 422.13 mm, what represents a length above the length of the first reproductive cycle (410.00 mm). The amplitude of selection goes from 265.27 to 578.99 mm, indicating that approximately 50% of fishes caught are adult. It is suggested, therefore, the use of "gill nets" with mesh sizes equal or bigger than 80 mm between opposite knots, to assist in the drafting of specific legislation, besides tracking of the selectivity and the average length of capture.